1. Field of the Invention
This invention relates to tachometers and more particularly to a digital tachometer including a photo-optical sensor for measuring the speed of a rotating or reciprocating object.
2. Description of the Prior Art
In many phases of industrial and commercial operations, it is frequently necessary to measure the rotational speed of machinery. Such measurement may be accomplished in a number of ways, depending upon the nature of the object to be measured. For example, a contact-type rotational speed measuring device, employing a revolution counter may be used, with the operator observing the number of revolutions of the rotating object occuring in a given interval of time and computing the rate per minute. Several types of hand held, contacting type rotational speed measuring devices are also available which enable direct reading of the speed of a rotating object without the need for making calculations.
One disadvantage of contact-type measuring instruments is that certain rotating mechanisms do not have an accessible contact point to which the measuring device can be attached, or even though the device may have a contact point available, the contact point may be nearly inaccessible. Moreover, the need to attach the sensing mechanism of the measuring apparatus directly to the rotating object creates an additional frictional force which is imposed on the rotating object, thereby hindering its operations.
The use of non-contacting type measuring instruments, such as the stroboscope, obviate most of the disadvantages of contact-type measuring instruments. However, the stroboscope also has limitations. For example, since the stroboscope functions on the principle of synchronization, the speed of the rotating object has to be constant. Thus, the operator has to synchronize the rate of a light source which goes on and off with the speed of the rotating object so that the object appears to be stationary. If the speed of the rotating object is changing, it is very difficult for the operator to keep track of it. In addition, because of harmonic relationship, synchronization may appear to exist at any one of several viewing rates and erroneous measurements may result. Also, since reading has to be taken from a divided scale, error of interpretation and interpolation is very likely to occur.
Thus, since the principle of operation of the stroboscope requires that rapidity of motion be greater than the response time of the human eye, the stroboscope cannot be used for low speed applications.
A further limitation is that the stroboscope usually requires a line outlet to derive its power, and accordingly, the stroboscope is not truly portable and its use is limited to locations where a power outlet is available.
Other contactless rotating speed measuring devices, such as the types using a photocell, a source of light, a hybrid type of circuitry and a current driven meter, are generally unsuitable for measuring at very low or very high speeds. On low speeds, the needle of the meter vibrates because the power pulses that drive the meter are far apart from each other, and there is a limit in the damping circuitry used to alleviate the vibration problem. On high speeds, the operator has to read divided scales and make some interpretations or interpolation of the reading. Moreover, the metering device itself has a percent error associated with it, limiting the accuracy of the measurement.
Therefore, it would be desirable to have an instrument for measuring rotational speed or reciprocating motion which overcomes the limitations mentioned above.